System and method for activating fuel cells

ABSTRACT

A system for activating a fuel cell includes a flow meter for measuring the amount of water discharged from an outlet of the air electrode and an outlet of the fuel electrode; a pressure sensor for measuring the pressure at the respective outlets; and a back pressure regulator receiving flow values measured by the flow meters and pressure values measured by the pressure sensors, which are fed back from a controller, and regulating a pressure difference (ΔP=P Cathode −P Anode ) to be a value greater than 0. With the system, the activation time of a fuel cell and the amount of hydrogen used for the activation can be reduced, thus improving the productivity and manufacturing cost.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional application of U.S. patent applicationSer. No. 12/503,271, filed Jul. 15, 2009, which claims under 35 U.S.C.§119(a) the benefit of Korean Patent Application No. 10-2008-0117428filed Nov. 25, 2008, the entire contents of which are incorporatedherein by reference.

BACKGROUND

(a) Technical Field

The present disclosure relates to a system and method for activating afuel cell. More particularly, it relates to a system and method foractivating a fuel cell, which can reduce the activation time of a fuelcell and reduce the amount of hydrogen used for the activation, thusimproving the productivity of a fuel cell stack and reducingmanufacturing cost.

(b) Background Art

A fuel cell stack after assembly is required to be activated beforebeing mounted on a vehicle. Otherwise, electrochemical reaction in thefuel cell does not occur to full extent during initial operation and theoverall performance of the fuel cell stack is irreversibly deteriorated.

The activation of the fuel cell provides advantageous effects including,e.g., removal of impurities introduced during the process ofmanufacturing the membrane-electrode assembly and the fuel cell stack,activation of a catalyst which does not participate in the reaction,creation of a transfer passage of reactants to the catalyst, andhydration of electrolyte contained in an electrolyte membrane and anelectrode to ensure a hydrogen ion passage.

As shown in FIG. 1, a conventional activation system includes anelectronic load 12 connected between a fuel electrode (“anode” or“negative electrode) and an air electrode (“cathode” or “positiveelectrode”) of a fuel cell stack 10, a pressure sensor 14 mounted on anoutlet of each of the fuel electrode and the air electrode, and acontroller 16 controlling the activation of the fuel cell stack 10.

According to a conventional method for activating a fuel cell, afterhumidified hydrogen and humidified air (oxygen) are supplied to the fuelelectrode and to the air electrode, respectively, activation accordingto a load sequence is initiated under predetermined operating conditions(stoichiometric ratio of fuel gas to air, relative humidity,temperature, and pressure). As an electrochemical reaction occurs in thefuel cell stack according to the load sequence, the amount of watercontained in a fluorine polymer electrolyte membrane is increased due towater produced at the air electrode and the humidified water supplied tothe fuel gas, thereby performing the activation.

One of the most important requirements for successful activation of thefuel cell stack is to control the percentage of water content at acertain level. That is, the concentration gradient of water contained inthe electrolyte membrane of the fuel cell stack must be small.

In an example, as shown in FIG. 2, the load is sequentially applied inthe order of (1) OCV (15 min)→(2) 600 mV/cell (75 min)→850 mV/cell (20min)→(4) 600 mV/cell (30 min) with the steps (3) and (4) repeated threetimes.

In another example, as shown in FIG. 3, the load sequentially applied intwo processes, namely, a pre-process and a post-process. The pre-processis performed in the order of 100→900 mV/cell (each 100 mV/cell−2min)→1,000 mV (30 min) and the post-process is performed in the order of900→100 mV/cell (each 100 mV/cell−5 min).

The above-described conventional methods, however, have the followingproblems. First, it takes a long time to perform the activation due tolimitations on utilization of product water of the fuel cell stack. Thatis, in the water transport according to an increase in load duringoperation of the fuel cell stack, the amount of water transported byelectro-osmotic drag at the fuel electrode becomes larger than theamount of water transported by back diffusion at the air electrode,causing only the water concentration on the surface of the electrolytemembrane at the air electrode to increase. Due to this, waterconcentration gradient in the electrolyte membrane occurs. It thus takesa long time to perform the activation and the productivity of the fuelcells stack is significantly reduced.

Here, since the Nafion fluorine-containing polymer electrolyte membranehas a hydrophobic PTEE structure on a surface layer thereof and ahydrophilic sulfonic acid structure in an inner layer thereof, a largeamount of humidified water supplied to the air electrode as fuel (air)is discharged as it is before it permeates into the inner layer. As aresult, a water concentration gradient occurs on the surface of theelectrolyte membrane at the fuel electrode and the air electrode. Inview of this, since the water produced in the fuel cell stack by thereaction is present in the inner layer of the electrolyte membranehaving hydrophilic properties rather than the humidified water suppliedas fuel, the product water is more advantageous for the activation thanthe humidified water.

Second, the amount of hydrogen used is increased. That is, since ittakes a long time to activate the fuel cell stack, the amount ofhydrogen fuel used is increased, and thus the cost for activating thefuel cell stack is increased.

The above information disclosed in this Background section is only forenhancement of understanding of the background of the invention andtherefore it may contain information that does not form the prior artthat is already known in this country to a person of ordinary skill inthe art.

SUMMARY OF THE DISCLOSURE

In one aspect, the present invention provides a system for activating afuel cell, in which an electrode load connected between a fuel electrodeand an air electrode of the fuel cell stack and a controller controllingactivation of the fuel cell stack are provided. The system may comprisea flow meter, a pressure sensor, and a back pressure regulator. The flowmeter, the pressure sensor, and the back pressure regulator are mountedon the outlet of the air electrode and the outlet of the fuel electrode.

The flow meter measures the amount of water discharged from an outlet ofeach of the air electrode and the fuel electrode. The pressure sensormeasures the pressure at the outlet of each of the air electrode and thefuel electrode. The back pressure regulator receives flow valuesmeasured by the flow meters and pressure values measured by the pressuresensors, which are fed back from the controller, and regulates apressure difference between the air electrode and the fuel electrode tobe a value greater than 0.

In another aspect, the present invention provides a method foractivating a fuel cell, which comprises: initiating activation of a fuelcell stack by supplying hydrogen and air (oxygen) to a fuel electrodeand an air electrode, respectively, under predetermined operationconditions; applying a predetermined load sequence to the fuel cell;regulating a pressure difference (ΔP=P_(Cathode)−P_(Anode)) between theair electrode and the fuel electrode to be a value greater than 0; andterminating the activation when the amounts of water discharged from thefuel electrode and the air electrode are equal to each other and thecurrent density of the fuel cell stack is stabilized at a constantlevel.

Other aspects and features of the invention are discussed infra.

It is understood that the term “vehicle” or “vehicular” or other similarterm as used herein is inclusive of motor vehicles in general such aspassenger automobiles including sports utility vehicles (SUV), buses,trucks, various commercial vehicles, watercraft including a variety ofboats and ships, aircraft, and the like, and includes hybrid vehicles,electric vehicles, plug-in hybrid electric vehicles, hydrogen-poweredvehicles and other alternative fuel vehicles (e.g. fuels derived fromresources other than petroleum). As referred to herein, a hybrid vehicleis a vehicle that has two or more sources of power, for example bothgasoline-powered and electric-powered vehicles.

The above and other features of the invention are discussed infra.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features of the present invention will now bedescribed in detail with reference to certain exemplary embodimentsthereof illustrated the accompanying drawings which are givenhereinbelow by way of illustration only, and thus are not limitative ofthe present invention, and wherein:

FIG. 1 is a schematic diagram showing a conventional system foractivating a fuel cell;

FIGS. 2 and 3 are graphs illustrating a conventional method foractivating a fuel cell;

FIG. 4 is a schematic diagram showing a method for activating a fuelcell in accordance with the present invention;

FIG. 5 is a flowchart illustrating a method for activating a fuel cellin accordance with the present invention; and

FIG. 6 is a graph comparing the activation times according to theconventional activation method and the activation method of the presentinvention.

Reference numerals set forth in the Drawings includes reference to thefollowing elements as further discussed below:

10: fuel cell stack 12: electronic load 14: pressure sensor 16:controller 18: back pressure regulator 20: hydrogen sensor 22: oxygensensor 24: flow meter

It should be understood that the appended drawings are not necessarilyto scale, presenting a somewhat simplified representation of variouspreferred features illustrative of the basic principles of theinvention. The specific design features of the present invention asdisclosed herein, including, for example, specific dimensions,orientations, locations, and shapes will be determined in part by theparticular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent partsof the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Hereinafter reference will now be made in detail to various embodimentsof the present invention, examples of which are illustrated in theaccompanying drawings and described below. While the invention will bedescribed in conjunction with exemplary embodiments, it will beunderstood that present description is not intended to limit theinvention to those exemplary embodiments. On the contrary, the inventionis intended to cover not only the exemplary embodiments, but alsovarious alternatives, modifications, equivalents and other embodiments,which may be included within the spirit and scope of the invention asdefined by the appended claims.

The present invention aims at providing a system and method foractivating a fuel cell which can adjust the amounts of water dischargedfrom an air electrode and a fuel electrode to be equal to each other bycontrolling a pressure difference between the air electrode and the fuelelectrode during activation of the fuel cell and prevent a crossoverphenomenon between air (oxygen) and hydrogen due to the pressuredifference between the air electrode and the fuel electrode.

A system for activating a fuel cell in accordance with an embodiment ofthe present invention for achieving the above objects is shown in FIG.4.

Referring to FIG. 4, the system comprises an electronic load 12connected between a fuel electrode (“anode” or “negative electrode) andan air electrode (“cathode” or “positive electrode”) of a fuel cellstack 10, a pressure sensor 14 mounted on an outlet of each of the fuelelectrode and the air electrode, a controller 16 controlling theactivation of the fuel cell stack 10, a back pressure regulator 18located at a position before the pressure sensor 14, a hydrogen sensor20, an oxygen sensor 22, and a flow meter 24, which are located inparallel after the pressure sensor 14.

That is, according to the present invention, the back pressure regulator18, the hydrogen sensor 20, the oxygen sensor 22, and the flow meter 24are further provided in addition to the pressure sensor 14 at the outletof each of the fuel electrode and the air electrode of the system foractivating the fuel cell.

Thus, the amounts of water discharged from the air electrode and thefuel electrode are measured by the flow meters 24 provided at theoutlets of the fuel electrode and the air electrode, the pressures atthe outlets of the fuel electrode and the air electrode are measured bythe pressure sensors 14, and the flow measurement values and thepressure measurement values are fed back to the controller 16 totransmit a signal to the back pressure regulator 18.

Then, the back pressure regulator 18 controls the pressure difference(ΔP=P_(Cathode)−P_(Anode)) between the air electrode and the fuelelectrode such that the amount of water transported by back diffusion atthe air electrode becomes larger than the amount of water transported byelectro-osmotic drag at the fuel electrode, thus adjusting the amountsof water discharged from the air electrode and the fuel electrode to beequal to each other.

At this time, a crossover phenomenon in which air (oxygen) at the airelectrode crosses over to the fuel electrode, or hydrogen at the fuelelectrode crosses over to the air electrode may be caused by thepressure difference (ΔP=P_(Cathode)−P_(Anode)) between the air electrodeand the fuel electrode.

When the air (oxygen) and hydrogen that have crossed over are dischargedthrough the outlets of the fuel electrode and the air electrode togetherwith water, the hydrogen and oxygen sensors 20 and 22 detect the sameand, if the detected values exceed threshold values, the operation ofthe back pressure regulator 18 is stopped as a warning (safety) step.

Moreover, the crossover of the gases such as hydrogen and air (oxygen)due to the pressure difference between the air electrode and the fuelelectrode can be checked by measuring a change in voltage in the systemfor activating the fuel cell.

In fact, when the water produced during the activation of the fuel cellpermeates into the electrolyte membrane, the crossover of the fuel gasesis extremely limited within a few ppm; however, the step of measuringthe amount of gases that have crossed over in the above manner andwarning the same by the hydrogen and oxygen sensors 20 and 22 isperformed to ensure the safety.

Meanwhile, the system and method for activating the fuel cell inaccordance with the present invention may be applicable to existingmethods for activating fuel cells such as an activation method withno-load humidification, a constant current activation method, a loadsequence activation method, etc.

For reference, the activation method with no-load humidificationactivates the fuel cell by supplying only humidified fuel to the fuelcell stack in a no-load state, the constant current activation methodoperates the fuel cell stack at a constant current, and the loadsequence activation method activates the fuel cell in such a manner thata load is maintained for a predetermined period of time or increased ateach current step from OCV (or low current) to high current and, then,the load is reduced at each current step from the maximum current toOCV. The load sequence activation method is most widely used.

A method for activating a fuel cell in accordance with an embodiment ofthe present invention based on the above-described system will bedescribed in detail with reference to FIG. 5 below.

FIG. 5 is a flowchart illustrating a method for activating a fuel cellin accordance with the present invention.

First, humidified hydrogen is supplied to the fuel electrode andhumidified air (oxygen) is supplied to the air electrode such that theactivation according to the load sequence is initiated underpredetermined operating conditions (stoichiometric ratio of fuel gas toair, relative humidity, temperature, and pressure).

At this time, the initial set values of fuel cell operating variables(stoichiometric ratio of fuel gas to air, relative humidity,temperature, and pressure) are input to the controller controlling theactivation of the fuel cell stack (S101).

Examples of the initial set value ranges are as follows:

-   -   1. Stoichiometric ratio of fuel gas to air        [Sr_(i(H2,Air))]:H2:Air=1 to 2:1.5 to 3;    -   2. Cell operation temperature of fuel cell stack [T_(i(cell))]:        55° C. to 95° C.;    -   3. Relative humidity of fuel gases [RH_(i(H2,Air))]: 25% to        100%; and    -   4. Pressure difference between air electrode and fuel electrode        (ΔP=P_(Cathode)−P_(Anode)): 0.1 to 2 bar.

Next, current values of the fuel cell operating variables(stoichiometric ratio of fuel gas to air, relative humidity,temperature, and pressure) are measured and compared with the initialset values (S102). If the measured values reach the initial set values,it is determined that the fuel cell operating variables are optimized,and thus the activation of the fuel cell stack is initiated.

On the contrary, if the measured values do not reach the initial setvalues, it is determined that the fuel cell operating variables are notoptimized, and thus the activation of the fuel cell stack is notinitiated until the respective variables reach the initial set values.

Here, an active load sequence for the activation is applied in steps atthe same time as the activation of the fuel cell stack is initiated(S104). For example, as shown in FIG. 5, the active load sequence may beapplied in three steps. That is, in the first step (Step 1), each cellvoltage is increased from 100 mV to 900 mV and maintained for 2 minutesat each increase of 100 mV. In the second step (Step 2), each cellvoltage is increased up to 1,000 mV and maintained for 30 minutes. Inthe third step (Step 3), each cell voltage is decreased from 900 mV to100 mV and maintained for 5 minutes at each decrease of 100 mV.

At this time, the pressure difference (ΔP=P_(Cathode)−P_(Anode)) betweenthe air electrode and the fuel electrode is maintained at a valuegreater than 0, preferably, within a range of 0.1 to 2.0 bar (S103) suchthat the amounts of water discharged from the fuel electrode and the airelectrode are adjusted to be equal to each other.

In more detail, the amounts of water discharged from the fuel electrodeand the air electrode are measured by the flow meters mounted on theoutlets thereof, and the pressure at the air electrode is increasedusing the back pressure regulator until the amounts of water dischargedfrom the fuel electrode and the air electrode are adjusted to be equalto each other.

In other words, the pressure difference (ΔP=P_(Cathode)−P_(Anode))between the air electrode and the fuel electrode is adjusted to 0.1 to 2bar such that the amount of water transported by back diffusion at theair electrode becomes larger than the amount of water transported byelectro-osmotic drag at the fuel electrode. As a result, the amounts ofwater discharged from the fuel electrode and the air electrode areadjusted to be equal to each other.

Thereafter, if it is determined that the amounts of water dischargedfrom the fuel electrode and the air electrode are equal to each other(A_(An-water)=A_(Ca-water)), the concentration changes in hydrogen andoxygen at the fuel electrode and the air electrode are equal to eachother [C_(f(An-O2))=C_(i(Ca-O2)) and C_(f(Ca-H2))=C_(i(An-H2))], and thecurrent density CD_(f) of the fuel cell stack is in steady state at aconstant level (S105), the activation is terminated.

In the even that there are concentration changes in hydrogen and oxygenat the fuel electrode and the air electrode, that is, if the valuesdetected by the hydrogen and oxygen sensors mounted on the outlets ofthe fuel electrode and the air electrode exceed threshold values, theoperation of the back pressure regulator is stopped as a warning(safety) step to reduce the pressure at the air electrode.

At this time, a crossover threshold value (limited range) of hydrogenand air, i.e., the crossover limited range of the oxygen concentrationat the fuel electrode and the hydrogen concentration at the airelectrode with respect to the Nafion fluorine-containing polymerelectrolyte membrane is determined to be a value within a range changedto 1 to 10% compared to the initial concentration.

As described above, while the activation is performed by applying theactive load sequence to the fuel cell stack, the pressure differencebetween the air electrode and the fuel electrode is controlled such thatthe amount of water transported by back diffusion becomes larger thanthe amount of water transported by electro-osmotic drag. Accordingly,the water produced at the air electrode is not discharged as it is, butis directly used in the activation of the Nafion fluorine-containingpolymer electrolyte membrane such that the water concentration gradientof the polymer electrolyte membrane is minimized, thus reducing theactivation time of the fuel cell stack.

The following examples illustrate the invention and are not intended tolimit the same.

EXAMPLE

A fuel cell stack with 6 cells, which as active area of 250 cm², wasprepared. Nafion fluorine-containing polymer electrolyte membrane wasused and Pt/C catalyst was used at the fuel electrode and air electrode.The fuel cell stack is operated in the following conditions: fuel cellstack temperature of 70° C.; atmospheric pressure; SR ratio of H₂:Air of1.2:1.8; and relative humidity of H₂:Air of 100%:100%.

A load sequence was applied as described with reference to FIG. 3. Thatis, the load sequence was divided into a pre-process and a post-process,in which the pre-process was performed in the order of 100→900 mV/cell(each 100 mV/cell−2 min)→1,000 mV/cell (30 min), and the post-processwas performed in the order of 900→100 mV/cell (each 100 mV/cell−5 min).

While the activation was performed by applying the active load sequenceto the fuel cell stack as described above, the pressure differencebetween the air electrode and the fuel electrode was adjusted to 0.1 to2.0 bar such that the amount of water transported by back diffusionbecame larger than the amount of water transported by electro-osmoticdrag, and thus the amounts of water discharged from the fuel electrodeand the air electrode were adjusted to be equal to each other.

Comparative Example

The fuel cell stack was prepared and activated in the same manner as theExample, except the pressure difference between the fuel electrode andthe air electrode was not adjusted.

In this Comparative Example, the amount of water transported byelectro-osmotic drag at the fuel electrode became larger than the amountof water transported by back diffusion at the air electrode, and thusonly the water concentration on the surface of the electrolyte membraneat the air electrode was increased.

Test Example

The activation time and the current density of the fuel cell stack atthe time point when the activation was terminated in accordance with theExample and Comparative Example were measured.

As a result, as shown in FIG. 6, 3.3 cycles were required until the timepoint when the performance of the fuel cell was stabilized at a currentdensity of 1,200 mA/cm² in the Comparative Example while only 1 cyclewas required in the Example.

As described above, the present invention provides various effectsincluding the following. During the active load sequence activation, thepressure difference between the air electrode and the fuel electrode iscontrolled such that the amount of water transported by back diffusionat the air electrode becomes larger than the amount of water transportedby electro-osmotic drag at the fuel electrode, thus adjusting theamounts of water discharged from the air electrode and the fuelelectrode to be equal to each other. As a result, the water produced atthe air electrode is not discharged as it is, but directly used in theactivation of a Nafion fluorine-containing polymer electrolyte membrane,such that the water concentration gradient of the polymer electrolytemembrane is minimized and, at the same time, the activation time of thefuel cell stack is reduced.

Moreover, with the reduction in the activation time of the fuel cellstack, it is possible to improve the productivity of the fuel cell stackand reduce the cost by reducing the amount of hydrogen gas used.

Furthermore, it is possible to prevent the crossover phenomenon due tothe pressure difference between the air electrode and the fuelelectrode, thus preventing deterioration of the fuel cell performancedue to carbon corrosion.

The invention has been described in detail with reference to preferredembodiments thereof. However, it will be appreciated by those skilled inthe art that changes may be made in these embodiments without departingfrom the principles and spirit of the invention, the scope of which isdefined in the appended claims and their equivalents.

What is claimed is:
 1. A system for activating a fuel cell, in which anelectrode load connected between a fuel electrode and an air electrodeof the fuel cell stack and a controller controlling activation of thefuel cell stack are provided, the system comprising: a flow metermeasuring the amount of water discharged from an outlet of each of theair electrode and the fuel electrode; a pressure sensor measuring thepressure at the outlet of each of the air electrode and the fuelelectrode; a back pressure regulator receiving flow values measured bythe flow meters and pressure values measured by the pressure sensors,which are fed back from the controller, and regulating a pressuredifference (ΔP=PCathode−PAnode) between the air electrode and the fuelelectrode to be a value greater than 0; and a hydrogen sensor and anoxygen sensor mounted on the respective outlets of the air electrode andthe fuel electrode and measuring the concentrations of hydrogen andoxygen discharged from the respective outlets of the air electrode andthe fuel electrode together with water, wherein the flow meter, thepressure sensor, and the back pressure regulator are mounted on theoutlet of the air electrode and the outlet of the fuel electrode.